The invention relates to energy device testing and evaluation.
A widely used technique for investigating the behavior of energy devices such as electrochemical cells or batteries is electrochemical impedance spectroscopy, also commonly known as frequency response analysis. See MacDonald, xe2x80x9cImpedance Spectroscopyxe2x80x9d, Wiley 1987. In general, the technique employs sinusoidal electric stimulation (AC voltage or current of known amplitude, frequency, and phase) of a device under test. Measuring the resultant in-phase and quadrature components of the device""s response allows calculation of the real and imaginary components of the device impedance using Ohm""s law (E=I*R or R=E/I). By taking a series of measurements over a range of frequencies, the characteristic response of the device under test is obtained. From the impedance parameters, other quantities, such as, for example, phase angle and modulus, may be derived.
Quantitative analysis is regularly achieved by using nonlinear least squares fitting to adjust parameters of a proposed theoretical model or electronic circuit analog. A well chosen model will correspond to the underlying chemical and kinetic processes. Frequency response analysis is very robust, but requires performance of multiple individual tests to obtain a complete frequency response profile. As such, the duration of the test process, especially if high resolution, low frequency response information is required, may be considerable. In addition, the time domain response of the device under test must be derived from a postulated equivalent circuit model, which means that accuracy of the derived response is strongly dependent on the validity of the model and the accuracy of the raw data.
To overcome these concerns, a number of direct time domain measurement techniques have been proposed. Commonly used techniques include voltammetry, polarography, chronoamperometry and chronopotentiometry, among others. The distinguishing characteristic of these time domain methods is that, instead of using a continuous sinusoid as the excitation signal, they use continuous (usually linear or exponentially shaped) segments separated by discontinuities. Typical stimuli include, for example: triangle or square waves; a rectangular pulse or pulse train followed by return to a pre-stimulus condition; a composite signal such as a stepped potential staircase with a smaller signal superimposed on each step; or, finally, a series of alternating charge/discharge events.
For general laboratory applications where theoretical analysis of reaction mechanisms is desired, time domain data may be analyzed using well known mathematical techniques, such as Fourier and Laplace integral transforms. The Fourier method allows derivation of a cell""s frequency response, while the Laplace method yields impedance and admittance information. When specifically applied to electrochemical accumulators (energy storage cells and batteries), time domain techniques may be used to assess cell condition and infer a relative state of charge.
The invention provides a highly accurate technique for measuring and analyzing electric potential changes occurring in a device such as an electric or electrochemical cell as a result of stimulation with a square wave current. An electric cell here is distinguished from an electrochemical cell in that voltage potential changes occurring in the electric cell are indicative primarily of charge storage and energy loss effects due to dielectric behavior (e.g., lossy capacitor), whereas potential changes observed in an electrochemical cell reflect additional processes including physical changes (mass transport, diffusion) and various Faradaic electrochemical reactions.
The technique uses a progressive change of the polarization voltage across a device, which develops over time in response to galvanic stimulation, as an estimator of device condition. Furthermore, suitable quantitative analyses of such changes in polarization, expressed as a joint function of stimulation magnitude, polarity and duration, allow quantitative characterization of various underlying chemical processes, identification of anomalous (fault) conditions, and estimation of state of charge.
The technique may be used to electronically measure devices that exhibit reversible or quasi-reversible reactions in response to a sufficiently small excitation signal symmetrically applied about the instantaneous equilibrium potential. While not all electrochemical systems have this property, a significant number of commercial applications require the precise measurement and characterization of just such devices. The technique can be used to evaluate the time domain response of any system which exhibits the property of electrical impedance.
The invention promises to help meet an ever growing need for reliable electrochemical devices that can deliver electricity on demand, and in many cases, be quickly and easily recharged for further use. Such devices include fuel cells, primary (single use) cells and batteries, and secondary (rechargeable) cells and batteries. There is a commensurate need for a technique for rapidly evaluating the state of charge and overall condition of such a device, regardless of whether the device is static (disconnected) or dynamically operating (charging/discharging).
The ability to rapidly perform a quantitative test of device condition is particularly important when the device is being used to supply power to a critical load, so that an unexpected failure may have serious consequences. Similarly, qualification testing during and immediately after batteries or other electrochemical cells are produced, the charge process would bring new economies to battery manufacturing. The same technique may be used in the field to perform tests prior to and after sale. Finally, the technique may be used in the laboratory to provide immediate information on electrochemical cell behavior under controlled conditions to support evolving battery technologies.
Polarization voltage is operationally defined in this document as the difference between the cell potential just prior to the onset of a step-wise change in current stimulation (i.e., the leading edge of a pulse or square wave) and the value attained at some specific later time during the stimulation. By employing high speed synchronous sampling methods, the actual waveform of the polarization voltage that develops during each half-period of the square wave excitation may be recorded for later analysis.
Specifically, a bipolar square wave current exhibiting a 50% duty cycle (mark-space ratio=1) and an average DC current component of exactly zero is used to stimulate an electrochemical cell or accumulator. The resultant polarization voltage response developed across the cell is repetitively sampled at a plurality of points at corresponding positions during each of the repetitions of the waveform. Piecewise numerical integration is performed by generating the sum of corresponding sample points from consecutive positive half cycles, and the separately the sum for the negative half cycles, respectively. These sums are then each divided by the number of samples (N) yielding an average value exhibiting a xe2x80x9cSquare Root of Nxe2x80x9d noise reduction factor. The relative shape and size of these averaged curves may be then analyzed or transformed as required to yield detailed information about the condition and future performance of the electrochemical cell (or battery). Data may be converted to digital format, using either a linear or exponentially spaced sampling algorithms. Linearly sampled data is suitable for processing with integral Laplace and Fourier transforms, while exponentially sampled data is useful for immediate graphical presentation of test results.
For chemical systems embodying reversible or quasireversible redox reactions, it has been determined that, when the galvanic stimulation takes the particular form of an even numbered sequence of square pulses of alternating polarity and of sufficiently small amplitude to ensure that cell response will be linear, the net charge transferred to the cell will be precisely zero. In addition, the resultant polarization voltage response will exhibit a characteristic symmetry about the time axis. Such a technique is, by definition, non-invasive. Any loss of symmetry detected in the polarization response is necessarily indicative of a breach of one of the three initial conditions, and thus may serve as an indicator thereof.
When this technique is used to test batteries, immediate assessment of condition and relative state of charge can be made by plotting deviations from the mean values. Mean value data is obtained from measurements of many known-good cells of the particular type. The deviations from these mean values obtained for the cell under test are then calculated to produce a fingerprint that can be interpreted visually for qualitative understanding.
Laplace transform techniques can be applied to calculated impedance parameters to construct models of time domain behavior for individual underlying processes, while the Fourier transform (and particularly the DFT) permits translation of the data into the frequency domain.
The technique facilitates rapid acquisition and analysis of energy cell state of charge and overall condition. The technique permits ease of use and efficient, portable operation. The technique also facilitates analysis of cell state of charge and condition. An apparatus according to the technique may include a self centering and polarizing circuit with respect to connection of test leads to a device under test that exhibits an intrinsic bias potential. To this end, the circuitry may include four connectors that include test leads each supplied with suitable connecting clamps, clips or fixturing. The connectors are affixed to terminals of the device under test, so that the connection for the non-inverting preamplifier input and the current drive signal are made to the same terminal of the device under test, while the connections for the inverting preamplifier input and the current receive signal are made to the opposite terminal of the device under test. This connection method is known as a Kelvin connection.
The technique may provide a symmetric current signal to a cell under test to produce a non-invasive test method. The technique may measure and report the voltage response of the cell under test to determine state of charge and cell condition, and may process the voltage response of a cell under test to remove from that signal a time invariant component to isolate a time variant component.
The technique may be used with different types of energy devices. For example, detection of asymmetry between positive and negative half-cycle responses may indicate a nonlinear transfer function, indicative, for example, of semiconductor diode behavior within an electrode, characterizing severe discharge in lead acid cells. Similarly, the relative age of lithium ion cells (number of charge/discharge cycles experienced) can be estimated from relative separation of polarization curves when calls are otherwise equated for open circuit voltage.
A time variant component of the voltage response of a cell under test may be sampled at periodic intervals to produce a linear representation of the signal. A time variant component of the voltage response of a cell under test may be sampled at logarithmic intervals to produce a logarithmic representation of the signal. Cell condition data may be acquired in an automated fashion under microprocessor control. Cell condition data may be acquired and stored in a format useable by an associated data processing device. A graphical transformation of cell voltage response data may be implemented to facilitate evaluation and analysis of state of charge and overall cell condition.
Repetitive test signal summation and integration techniques may be used to reduce the interference of ambient noise. Test signals of fixed frequency may be provided, as may be test signals of different frequencies. A well-defined test signal may be used to ensure evaluation of the cell under test at a wide range of waveform periods. The voltage response of a cell under test may be created and captured in a form to facilitate analytic evaluation of the cell using Fourier, Laplace and related techniques. Similarly, the voltage response of a cell under test may be created or captured in a form to facilitate evaluation by neural networks.
The technique permits rapid and accurate acquisition of information relevant to the state of charge and qualitative information about an electrochemical energy storage device, commonly referred to as a battery. The technique may provide a precision test instrument that produces precise driving signals to create polarization response voltages as a function of time which are developed across an electrochemical cell or battery of cells in response to a galvanic (current) stimulation, and may be captured to present data reflecting state of charge and cell quality information. The technique includes specific noise reduction and small signal detection and processing capabilities to permit the use of non-invasive driving signals. The technique further relates to acquiring and manipulating data to facilitate analysis and presentation of detailed information rapidly and efficiently.
The invention features determining polarization voltages developed in response to an excitation signal applied to a device. Such polarization voltages may be determined by using a controlled-current source configured for connection to a first terminal of the device, a controlled-voltage source configured for connection to a second terminal of the device, a sensor configured to sense a voltage across the device and to produce a sensor signal in response to the voltage, and a controller connected to the controlled-current source, the controlled-voltage source and the sensor. The controller is configured to determine polarization voltages in response to the sensor signal.
Embodiments of the invention may include one or more of the following features. The device may be an electrically-responsive element, network, electrochemical cell, or battery. The controlled-current source and the controlled-voltage source may be configured to provide self-centering and autopolarity relative to the device. To this end, the controller may be configured to provide self-centering relative to the device by supplying a voltage equal to one half of the bias voltage of the device to the controlled-voltage source. The controlled-current source may be configured to provide a symmetric, bipolar square wave to the first terminal of the device.
The controller may include a microprocessor and associated circuitry. Alternatively, the controller may be made up of analog circuitry.
Kelvin connection circuitry may be used to attach the components to the device. The Kelvin connection circuitry may include a first lead connected to the controlled-current source and configured for connection to the first terminal of the device, a second lead connected to the controlled-voltage source and configured for connection to the second terminal of the device, and third and fourth leads connected to the sensor and configured to be connected to, respectively, the first and second terminals of the device.
A feedback loop may be employed between the sensor and the controller. The feedback loop may be configured to eliminate a nonvarying or slowly varying portion of the voltage across the device (e.g., the bias voltage of the device) so that the sensor signal reflects only a portion of the voltage across the device.
In another general aspect, the invention features determining polarization voltages developed in response to an excitation signal applied to a device by connecting a controlled-current source to a first terminal of the device, connecting a controlled-voltage source to a second terminal of the device, and using a controlled-current source to apply a bipolar, symmetric square wave to the device. A voltage is sensed across the device to produce a sensor signal, and the sensor signal is modified to eliminate effects of a nonvarying or slowly varying portion of the sensed voltage. Polarization voltages are then determined in response to the modified sensor signal.
The controlled-voltage source may be controlled to produce a voltage having a magnitude equal to one half of the bias voltage of the device.
A graphical representation of the polarization voltage suitable for immediate visual inspection and analysis may be generated. The polarization voltage may be compared to baseline data for a class of devices to which the device belongs to assess a relative condition of the device.
Polarization voltage data obtained through testing with a symmetric bipolar square wave can manipulated in a fashion advantageous for a particular form of graphic representation suitable for immediate visual inspection and analysis. The polarization voltage data also can be used to assess the relative condition of an electrochemical cell or battery. Polarization response profiles are obtained for the device under test and compared to baseline data stored in memory. Specific differences in the shapes of the response patterns are indicative of specific problems within the device under test and, for specific types of devices under test, only certain data points, corresponding to specific polarization frequencies, need be evaluated to make the necessary determination.
A DC offsetting signal may be permitted to vary during the test period, such that polarization voltage data may be obtained through testing with a symmetric bipolar square wave concurrently with DC charging or discharging current being supplied to the device under test.
Changes in the polarization voltage of a cell can be used to accurately determine the end-of-charge point, which is reached when at least one of the chemical species needed for the recharge reaction has been effectively depleted, whereupon the equivalent Faradaic resistance (characteristic of the electrode-electrolyte interface) commences a precipitous increase that is manifested as a commensurate increase in the measured polarization voltage, and is especially apparent at relatively low equivalent polarization frequencies in the range of 1 to 0.01 Hertz. When such a rise is detected, it may be used as an indication of end-of-charge, and so employed as a stopping signal for a concurrent charging excitation.
The waveforms produced may be arbitrary. That is, the waveforms may be comprised of sine waves, triangle waves, ramps, pulses, complex parametric shapes or any combination thereof, in order to assess the various time dependent characteristics of galvanically stimulated cells and electrical networks.
The time dependent evolution of polarization voltages may be measured according to various digital sampling time schedules, such as those characterized as a constant xcex94T schedule, wherein the time elapsed between successive samples is held constant; a logarithmic xcex94T schedule, wherein the time elapsed between successive samples is increased in an exponential manner; or a parametric xcex94T schedule, wherein the time elapsed between successive samples is determined by a set of values programmed into the prestored software. Nonlinear sampling schedules, when properly constituted to accurately capture the polarization events of interest for a particular chemistry, confer the advantage of a considerable reduction in data collection and storage requirements. In the ultimate case, only a very few data points may be required, allowing the entire method to be incorporated into a single, relatively simple, integrated circuit.
Other features and advantages of the invention will become apparent from the following description, including the drawings, and from the claims.